Crystallinity indices of quartz in chert of Central Japan: their reliability and some geological applications

概要

論文の詳細を見る
Crystallinity indices of quartz(CI) determined by X-ray analysis have been reported in various types of chert of different ages. Reported CI's distribute from <1 to 10. The difference may result from the difference in sample state prepared for determination of CI or existence of essential variation in crystallinity of quartz. We determined CI of many euhedral quartz crystals and quartz in cherts in different sample states, and could conclude that CI does not depend largely on sample states prepared with different methods, but depends partially on euhedral macroquartz used as a standard material. Even after removing this disadvantage, there is still wide difference in CI of quartz, indicating that CI is not uniform but varies according to the mode of origin and the thermal processes having operated on the quartz. Our data coupled with data described in literature show that 1) CI's in Cenozoic deep-sea oozes and cherts are lower than those in Mesozoic bedded chertz on land, 2) bedded cherts suffering thermal effect are characterized by higher CI, 3) CI's lacustrine and fluvial cherts are higher than those of bedded cherts of deep-sea origin, 4) CI's of older cherts are generally larger than those of younger cherts of the similar origin.This analysis suggests that CI can be used as a useful indicator deciphering origin of sedimentary siliceous rocks and their diagenetic and metamorphic stages.